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An interpretation of LIEBIG's "law of the minimum", LIEBSCHER's and WOLLNY's "law of the optimum" and MITSCHERLICH'S "law of constant activity" for two growth factors is presented. With three-quadrant diagrams (crop yield vs. N fertilization, crop yield vs. N uptake, N uptake vs. N fertilization) a long-term field experiment in Göttingen (Ackerbau-Systemversuch) is analyzed. This experiment started 1981 with the following treatments: 1. crop rotation (a: short rotation, sugar-beet, winter-wheat, winter-barley/vetchgreen- manure; b: long rotation, sugar-beet, winter-wheat, winter-barley, lucerne, maize, faba beans); 2. chemical plant protection (a: with; b: without); 3. mineral nitrogen fertilization procedure (a: N-fixed, cumulative effects; b: N-rotating, one year's effects of different N fertilization); 4. mineral nitrogen fertilization (a: nil; b: suboptimal; c: optimal; d: hyper-optimal doses). Without chemical plant protection resource use efficiency of applied nitrogen decreased according to LlEBSCHER's and WOLLNY'S "law of the optimum". In most cases this "law of the optimum" was applicable to the empirical data. LIEBIG's "law of the minimum" and MITSCHERLICH's "law of constant activity" are exceptional cases, whereas LiEBSCHER's and WOLLNY'S "law of the optimum" has more general validity. For maximal resource use efficiency the growth factors have to be combined in optimal amounts. [ABSTRACT FROM AUTHOR]